In this specification, abbreviated names of halogenated hydrocarbon compounds are described in brackets after the compound names, and in this specification, the abbreviated names are employed instead of the compound names as the case requires.
Heretofore, as a working fluid for a heat cycle system such as a refrigerant for a refrigerator, a refrigerant for an air-conditioning apparatus, a working fluid for power generation system (such as exhaust heat recovery power generation), a working fluid for a latent heat transport apparatus (such as a heat pipe) or a secondary cooling fluid, a chlorofluorocarbon (CFC) such as chlorotrifluoromethane or dichlorodifluoromethane or a hydrochlorofluorocarbon (HCFC) such as chlorodifluoromethane has been used. However, influences of CFCs and HCFCs over the ozone layer in the stratosphere have been pointed out, and their use is regulated at present.
Under the above conditions, as a working fluid for a heat cycle system, a hydrofluorocarbon (HFC) which has less influence over the ozone layer, such as difluoromethane (HFC-32), tetrafluoroethane or pentafluoroethane (HFC-125) has been used. However, although HFCs have less influence over the ozone layer, it is pointed out that they may cause global warming since they have a high global warming potential (hereinafter referred to as GWP). Accordingly, as a working fluid for a heat cycle system, development of a working fluid having little influence over the ozone layer and having a low GWP is urgently needed.
In recent years, as a working fluid for a heat cycle system, a hydrofluoroolefin (HFO) having a carbon-carbon double bond which is likely to be decomposed by OH radicals in the air and thereby having less influence over the ozone layer, and having a low GWP, has been proposed.
As a working fluid employing such a HFO, for example, Patent Document 1 discloses a technique relating to a working fluid using 1,1,2-trifluoroethylene (HFO-1123) which has the above properties and with which excellent cycle performance will be obtained. Patent Document 1 also discloses an attempt to obtain a working fluid comprising HFO-1123 and various HFCs or HFOs in combination for the purpose of increasing the flame retardancy, cycle performance, etc. of the working fluid.
However, HFO-1123 may undergo self-decomposition reaction under high temperature and high pressure conditions, and if a composition containing HFO-1123 is practically used, improvement of durability of a working fluid using HFO-1123 should be achieved.
Further, HFO-1123 is a compound having an unsaturated bond in its molecule and is a compound having a very short life in the air, and accordingly under conditions under which compression and heating are repeatedly carried out in a heat cycle, it is inferior in the stability to saturated HFCs and HCFCs which are conventional HFCs and HCFCs in some cases.
Under these circumstances, a method for efficiently operating a heat cycle system employing HFO-1123 as a working fluid, with improved stability while excellent cycle performance of HFO-1123 is sufficiently made use of, has been desired.